Responsiveness for a given concentration of sucrose is influenced by both the absolute and relative stimulus properties of the tastant. If there is no opportunity for comparison between two disparate concentrations of sucrose, a rat will respond to the absolute stimulus properties of each tastant by generating more licks for a high than for a low concentration of sucrose. This is referred to as a concentration effect. If, on the other hand, the rat is given the opportunity for comparison, licking behavior also will be influenced by relative stimulus properties. That is, intake of the low concentration will be suppressed and, intake of the high concentration enhanced, relative to non-comparison conditions. This exaggerated response, resulting from the immediate comparison of two different concentrations of sucrose, is referred to as a Simultaneous Contrast Effect. Simultaneous Contrast is the behavioral expression of the most basic form of comparison examined in the gustatory system. It is believed to primarily reflect sensory processes, rather than the more complex long-term memory or associative processes thought to mediate the two other forms of consummatory contrast. While both the absolute and relative stimulus properties of a tastant contribute to ingestion, scientific investigation has focused almost exclusively on absolute stimulus properties. No attempt has been made to directly evaluate the neural mechanisms that mediate processing of the relative stimulus properties of a tastant. Some evidence suggests that a normal concentration-response function for sucrose reflects both absolute and relative stimulus properties and that the isolated brainstem is both necessary, and sufficient, to support this behavior. Thus, it seems logical to begin the evaluation of the neural mechanisms mediating responsiveness to the relative stimulus properties of a tastant (i.e., Simultaneous Contrast Effects) at the level of the first and second gustatory relays in the brainstem. The present experiments are designed (1) to examine the behavioral expression of Simultaneous Contrast Effects in rats using an automated gustometer, (2) to evaluate the contribution of the first and the second central gustatory relay in Simultaneous Contrast in the gustometer using lesion techniques and, (30 to ascertain the extent to which the brainstem may, in and of itself, be sufficient for the occurrence of contrast by using a decerebrate preparation.